An inlet casing is where a motive fluid, such as steam, initially comes into a turbine. Typically, the inlet casing has one or more passages which direct the motive fluid to a first stage stationary component, such as a nozzle ring. The nozzle ring is a ring shaped plate attached to the downstream face of the inlet casing directly upstream of a first stage rotational component. The nozzle ring contains nozzle passages which accelerate and redirect the flow of the motive fluid. The first stage rotational component is a wheel with a plurality of buckets, termed a bucket row, attached to its outer circumference. The wheel of each stage is co-axially mounted to a shaft which rotates about a first axis, also referred to as the rotational axis of the turbine, and is positioned to receive the motive fluid from the passages in the nozzle ring.
Typically, casting patterns and tooling molds are used to form one or more castings which comprise the inlet casing with distribution ports. If the shape is too complex, then multiple castings are bolted or welded together to form the inlet casing. The inlet casing is then adapted to the particular turbine application. Alternatively, a fabrication of plates and tubes is welded and fitted together to form the inlet casing. This alternative method is labor intensive and time consuming.
Ideally, each inlet casing would be customized for each turbine application. Unfortunately, the process of redesigning tooling to make a customized casting for an inlet casing for each particular turbine application or to weld and fit plates together to form an inlet casing is time consuming and expensive. As a result, rather than manufacturing a new inlet casing, existing cast inlet casings with less than ideal dimensions are used which ultimately reduces the performance and efficiency and increases the cost of the turbine. To try and accommodate the various turbine applications in a timely manner, manufacturers often carry large inventories of pre-cast inlet casings or casing component castings which is an expensive practice.